Test of a method for monitoring the geostrophic meridional overturning circulation using only boundary measurements

Measurements of ocean bottom pressure, particularly on the continental slope, make an efficient means of monitoring large-scale integrals of the ocean circulation. However, direct pressure measurements are limited to monitoring relatively short time scales (compared to the deployment period) because of problems with sensor drift. We use measurements from the northwest Atlantic continental slope, as part of the RAPID West Atlantic Variability Experiment, to demonstrate that the drift problem can be overcome by using near-boundary measurements of density and velocity to reconstruct bottom pressure differences with accuracy better than 1 cm of water (100 Pa). This accuracy permits us to measure changes in the zonally-integrated flow, below and relative to 1100 m, to an accuracy of 1 sverdrup or better. The technique employs the ``stepping method''---a generalization of hydrostatic balance for sloping paths that uses geostrophic current measurements to reconstruct the horizontal component of the pressure gradient.